- Expansion cohort 2 patients: If patient is RS+ (defined as having ≥ 15% ring sideroblasts in the bone marrow), no prior ESA treatment and serum erythropoietin level ≤ 200 U/L. If a patient is RS- (defined as having < 15% ring sideroblasts in the bone marrow), prior ESA treatment and any serum erythropoietin level is allowed.